The use of ionized gases (plasma) for treating, modifying and etching of material surfaces is well established. Both vacuum-based plasmas and those that operate at or near atmospheric pressure, have been used for surface modification of materials ranging from plastic wrap to non-woven materials and textiles, the plasma being used to provide an abundant source of active chemical species, which are formed inside the plasma, from the interaction between resident electrons in the plasma and neutral or other gas phase components of the plasma. Typically, the active species responsible for surface treatment processes have such short lifetimes that the substrate must be placed inside the plasma (“in-situ” processing). Thus, the substrate, and at least one stable “precursor” gas are present together inside a process chamber in contact with the plasma ranging in excitation frequencies from DC to microwave frequencies so that the short-lived active chemical species generated by the plasma are able to react with the substrate before decay mechanisms, such as recombination, neutralization or radiative emission can de-activate or inhibit the intended surface treatment reactions.
In addition to vacuum-based plasmas, there are a variety of plasmas that operate at or near atmospheric pressure. Included are dielectric barrier discharges (DBDs), which have a dielectric film or cover placed on one or both of the powered and ground electrodes (which may be planar or annular in design); corona discharges, which typically involve a wire or sharply-pointed electrode; micro-hollow discharges, which consist of a series of closely-packed hollow tubes that form either the rf or ground electrode and is used with a counter electrode to generate a plasma; a “flow-through” design, which consists of parallel-placed screen electrode and in which a plasma is generated by the passage of gas through the two or more screen electrodes; plasma jets in which a high gas fraction of helium is used along with electrical power in the 2 MHz-100 MHz range and a close electrode gap to form an arc-free, non-thermal plasma; and a plasma “torch”, which uses an of an arc intentionally formed between two interposed electrodes to generate extremely high temperatures for applications such as sintering, ceramic formation and incineration.
The use of atmospheric pressure gases for generating a plasma provides a greatly simplified means of treatment for large or high volume substrates, such as plastics, textiles, non-wovens, carpet, and other large flexible or inflexible objects, such as aircraft wings and fuselage, ships, flooring, commercial structures. Treatment of these substrates using vacuum-based plasmas would be complicated, dangerous and prohibitively expensive. The present state of the art for plasmas operating at or near atmospheric pressure also limits the use of plasma for treatment of these commercially-important substrates.
Of the various atmospheric pressure plasmas, the Dielectric Barrier Discharge (DBD) is the most widely used, and is characterized by the use of a dielectric film or cover on one or both of the electrodes to prevent formation of a persistent arc that would otherwise form between the electrodes. By accumulating charge on the surface of the dielectric as an arc forms, this build-up of charge acts to quench the arc, which typically reforms elsewhere on the electrode. The substrate itself may function as the dielectric cover, provided that it fully covers the exposed electrode. In some situations, a high gas fraction (>50%) of helium is added to the process gas to help homogenize the discharge. DBDs have the advantage of having a large gap between the electrodes, so that a thick substrate can readily be placed on one of the electrodes. However, since electrical power must be transmitted through the dielectric cover, the power density that a DBD discharge can achieve is limited. Low power density typically produces slow processing, because low-power density in the plasma also results in a slow generation rate of the active, chemical species responsible for materials processing. The dielectric cover on the electrode also inhibits heat removal since most electrical insulators also function as thermal insulators. Because of this, the gas temperature inside a DBD can often reach temperatures as high as 100° C.-200° C. during prolonged plasma operation (See, e.g., T. Stegmaier et al. et al., Plasma Technologies for Textiles ed. R. Shishoo, Woodhead Publishing, 2007, pg 140).
Corona discharges are also widely used for surface treatment and activation. In these discharges, a high electric field is generated in the vicinity of a wire or other electrode having sharply pointed edges. If the electric field is sufficient to remove electrons from neutral gas species, then ionization localized around the wire will result. Such plasmas are typically used for surface modification reactions, such as plastic food wrap. Again, slow processing speed and inability to cool the neutral gas temperature are characteristic of these discharges, and processing is limited to the treatment of dry, non-conductive substrates. There is no effective way to water-cool a wire electrode, because the means of plasma generation depends on the strong electric field created by a small radius of curvature.
Micro-hollow discharges, such as those described in U.S. Pat. No. 6,346,770 and U.S. Pat. No. 6,072,273 are characterized by series of micro-hollow openings in a conductive electrode that is typically covered with a dielectric layer. Each of the micro-hollow openings in the electrode has a cross-sectional area that is on the order of the mean free path of the electrons in the gas. Micro-hollow discharges are often operated at sub-ambient pressure, in the range of 0.1 to 200 Torr, but may also be operated at pressures approaching atmospheric pressure. The diameter of each opening in the electrode is in the range between 0.1 and 10 Torr-centimeters. For atmospheric pressure, this would equate to an opening having a diameter in the range of between 1.3 to 130 μm. The spacing between adjacent ground electrode tubes in this invention is preferably in the range of between 0.06 and 0.100 in. (1.5 mm to 2.54 mm) which, when multiplied by atmospheric pressure (760 Torr), equates to 114 to 193 Torr-cm, or more than 10× the guideline specified by the micro-hollow discharge. Further, the use of a dielectric cover on the electrode of the micro-hollow discharge reduces the ability to effectively cool the neutral gas temperature by water cooling of the electrodes, because thermal conduction is inhibited by the presence of the dielectric. Another problem for large area processing, is that the active treatment region is limited to the open area provided by each of the micro-hollow discharge elements.
The atmospheric-pressure plasma jet (APPJ) uses a process gas mixture consisting of >95% helium, electrical energy between 1 MHz and 100 MHz and a narrow gap between two conducting electrodes to achieve a stable, non-arcing plasma. The electrodes may be planar and parallel (U.S. Pat. No. 6,262,253), or annular in design (U.S. Pat. No. 5,961,772), but must have a uniform gap between the rf and ground electrodes. The use of helium gas mixtures with an electrode gap in the range of between 0.5 mm and 2.5 mm has been found to assist in the prevention of arcing when appropriate high frequencies are used to power the electrodes. Gas flow may be either along the longitudinal axis of the electrodes for the annular design, or may be along the planar axis for the parallel plate design. Certain substrates may be placed inside the discharge for treatment, provided the thickness of the substrate does not induce arcing, and the substrate does emit volatile gases that change the gas chemistry inside the discharge. The advantages of this design over other atmospheric pressure discharges are the ability to generate a large-area discharge having high-power density suitable for fast processing, and the ability to efficiently cool the neutral gas temperature since dielectric covers are not required, and since the use of solid metal electrodes permits internal water cooling to efficiently remove heat from the gases in the plasma.
An annular APPJ discharge apparatus where the gas was flowed between the rf and ground electrodes through a series of perforations in one of the uncooled electrodes has been used in a cleaning process (U.S. Pat. No. 6,228,330).
A flow-through electrode design using a gas flow consisting of predominately helium feed gas that flows through two metal screens that function as electrodes, one rf-powered and the other grounded is described in U.S. Patent Publication No. 2002/0129902. The discharge is created in the gap between the parallel, screen electrodes, which generally has the same spacing as the inter-electrode gap of the APPJ discharge. High gas flow rates through a large open area are required since the active chemical species must transit the distance between the point of creation in the plasma and the substrate which may be located several millimeters from the closest perforated electrode, thereby necessitating a fast linear flow rate. Further, the metal screen cannot be water-cooled, leading to a high, neutral gas temperature (>150° C.), especially if high rf power is used since heat removal is limited to conduction at the point of contact with the housing, and from the heat capacity of the gas as it exits the plasma.
A plasma torch is a thermal discharge characterized by generation of extremely high temperatures, often in excess of 10,000° C., which is destructive to substrates.